Fractal Evolution of Acoustic-Emission Dynamics in Green Sandstone Subjected to Wetting–Air-Drying Cycles: Correlation Dimension and Failure-Mode Transition

Wetting–air-drying cycling significantly alters the internal damage evolution and failure behavior of sandstone, and identifying reliable acoustic-emission (AE) precursors during loading is important for understanding the rupture mechanism of water-affected rock. In this study, uniaxial compression tests with AE monitoring were conducted on green sandstone subjected to different numbers of wetting–air-drying cycles. Ringing counts, RA–AF parameters, b-value evolution, AE spatial localization, and the correlation dimension D2 were jointly used to characterize mechanical deterioration, failure-mode transition, and fractal dynamic evolution. The results show that increasing cycling causes a progressive decrease in peak stress and elastic modulus, while AE activity evolves from a relatively dispersed state to stronger pre-peak concentration. The RA–AF distributions indicate that the dominant AE population gradually shifts from tensile-feature dominance toward mixed/shear-involved behavior, suggesting increasing shear participation during failure. The b-value captures stage-dependent damage evolution but exhibits relatively strong fluctuations under increasingly nonstationary event distributions. In contrast, D2 shows a clearer pre-peak turning feature, and the corresponding stress level remains relatively consistent among different cycling groups. These results indicate that wetting–air-drying cycling not only accelerates the mechanical degradation of green sandstone, but also substantially modifies its rupture dynamics. The D2 feature may therefore serve as a potential precursor parameter for characterizing pre-peak complexity transition in water-affected sandstone.